Eighteen on-farm demonstrations on System of Rice Intensification (SRI) were carried out in  ten  hectares of farmers fields in Sivagangai and Madurai districts of Tamil Nadu from October 2010 to February 2011 under Tamil Nadu-Irrigated Agriculture Modernization and Water Bodies Restoration and Management Project  (TN–IAMWARM) to study the effect and adoption level of rotary weeding on yield of rice under SRI. The results revealed that SRI registered a mean grain yield of 6.06 t /ha against 54.2 t /ha under conventional method of rice cultivation. SRI yielded 11.06 per cent higher grain yield than conventional method. The number of rotary weeding also decides the rate of yield increase in SRI. The average yield increment by four, thrice and twice rotary weeding under SRI over conventional method were 24.1, 15.4 and 8.5 per cent, respectively. Among the total farmers,  only 11.1 per cent farmers perfectly carried out four times rotary weeding as per recommendation. The percentage of farmers adopted thrice, twice and single rotary weeding were 44.5, 33.3 and 11.1 per cent, respectively. Adoption of SRI drastically reduced the cost of weeding as evident due to Rs 2, 534 /ha lesser weed management cost  under SRI  (Rs 2989 /ha) than conventional method of rice cultivation (Rs 5, 523/ha). The cost of cultivation was comparatively lesser in SRI which resulted in gaining an additional net profit of ` 11, 021 /ha as compared to conventional method of rice cultivation.